Heart-shaped mesa on surface of planet Mars in image made by Mars Global Surveyor Orbiter.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Heart-shaped mesa on surface of planet Mars in image made by Mars...

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Scientists discovered a valentine from Mars when a camera on the Mars Global Surveyor shot a heart-shaped pit, formed when a straight-walled trough collapsed. The pit is about 1.4 miles (2.3 kilometers) at its widest. The photo was taken by the Mars Orbiter Camera on the Mars Global Surveyor.

Photo: NASA, Getty Images

Scientists discovered a valentine from Mars when a camera on the...

Scientists discovered a valentine from Mars when a camera on the... Photo-6784306.92462 - SFGate

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Scientists discovered a valentine from Mars when a camera on the Mars Global Surveyor shot a heart-shaped pit, formed when a straight-walled trough collapsed. The pit is about 1.4 miles (2.3 kilometers) at its widest. The photo was taken by the Mars Orbiter Camera on the Mars Global Surveyor.

Photo: NASA, Getty Images

Scientists discovered a valentine from Mars when a camera on the...

Scientists discovered a valentine from Mars when a camera on the... Photo-6784307.92462 - SFGate

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The "Smiley Face" - Crater Galle, lying to the east of the Argyre Planitia impact basin and south-west of the Wirtz and Helmholtz craters, at 51degrees South latitude and 329 degrees East longitude. The image clearly shows a large stack of layered sediments forming an outcrop in the southern part of the crater. Several parallel gullies, possible evidence for liquid water on the Martian surface, originate at the inner crater walls of the southern rim.

The "Smiley Face" - This image, obtained by the High Resolution Stereo Camera (HRSC) on the European Space Agency's Mars Express, is a mosaic of overlapping images gathered during five separate orbits. The ground resolution ranges between 10-20 metres per pixel, depending on location within the image strip, and the crater is shown lying near 51degrees South and 329 degrees East. North is up. The image shows Crater Galle containing a large stack of layered sediments forming an outcrop in the southern part of the crater. Several parallel gullies, possible evidence for liquid water on the Martian surface, originate at the inner crater walls of the southern rim. (Photo by SSPL/Getty Images)

Photo: Science & Society Picture Librar, SSPL Via Getty Images

The "Smiley Face" - This image, obtained by the High Resolution...

The "Smiley Face" - This image, obtained by the High... Photo-6784309.92462 - SFGate

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Canals on the planet Mars as drawn by Italian astronomer Giovanni Schiaparelli, Percival Lowell also made drawings of these canals and popularized the belief that these markings showed that Mars sustained intelligent life forms. Illustration. Author Camille Flammarion.

Plate from 'Les Terres du Ciel' (The Worlds of the Sky) written by the French astronomer Camille Flammarion (1842-1925). The plate is an artist's impression of how canals on Mars might have looked. The idea that there might be canals on Mars originated with the Italian astronomer Giovanni Schiaparelli, who claimed to have observed features he described as canali (channels) in 1877. Flammarion was not the only supporter of the idea; the respected American astronomer Percival Lowell (1855-1916) theorised that a civilization, faced with climate change, had built the canals to transport water from the planet?s ice caps to irrigate crops. In fact the canals did not exist; they are now thought to have been optical illusions possibly caused by dust storms.

Colour print by Etienne Leopold Trouvelot (1827-1895), one of a set of 15, made by him whilst in the United States between 1872 and 1881. The lithographs are based on the French artist's observations from various American observatories. Titled 'The Planet Mars', the print shows the appearance of the Red Planet on 3 September 1877. During that year, Mars was well paced for observation as it came especially close to the Earth, appearing as a bright red star in the night sky. This pastel drawing shows the prominent feature 'Solis Lacus' also known as the 'Eye of Mars'. Today, this dark area is known to be a plain and is named Solis Planum.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

Colour print by Etienne Leopold Trouvelot (1827-1895), one of a set...

A drawing of the solar system showing Mercury, Venus, the Earth, Mars, Jupiter and Saturn. Below, a man and a woman in a garden. Image taken from Conversations on the plurality of worlds Translated from the last Paris edition by W. Gardiner. Originally published/produced in 1715.

Photo: British Library/Robana, British Library/Robana Via Getty

A drawing of the solar system showing Mercury, Venus, the Earth,...

A drawing of the solar system showing Mercury, Venus, the Earth,... Photo-6784315.92462 - SFGate

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Plate of 1531 illustrating the planet Mars and those under its influence.

Photo: Universal History Archive, UIG Via Getty Images

Plate of 1531 illustrating the planet Mars and those under its...

Plate of 1531 illustrating the planet Mars and those under its... Photo-6784316.92462 - SFGate

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Aquatint print (monochrome) by W Forrest showing two views of the planet Mars from original drawings by Captain William Stephen Jacob (1813-1862). His sketches are base on his views of the planet seen on the 18 and 23 March 1854. For these observations Jacob used the 6 inch aperture telescope of the Madras Observatory made by Lerebours & Secretan of Paris. Jacob was director of the Indian Observatory from 1848-1862, a time during which he was re-equipped it extensively. Due to the southerly latitude of Madras, Mars was high in the sky and particularly well placed for detailed examination. By the end of the nineteenth century there was great speculation as to whether life existed on Mars.

Two Viking spacecraft were launched towards Mars by NASA in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms. No evidence of life was found, but more recent studies of the Martian landscape suggest that in the past surface water may have been present, which could have enabled life to exist. The Orbiters often observed ice clouds early in the morning around and within the Valles Marineris, a system of canyons some 4000 kilometres long and over 6 kilometres deep in places. Water ice is found at the Martian polar regions, together with frozen carbon dioxide (dry ice).

This view was taken by the Viking Orbiter 1 spacecraft. These canyons are some 4000 kilometres long and are over 6 kilometres deep in places. They are thought to have been formed by the Martian crust being stretched apart by the formation of another large landform, the Tharsis Bulge, rather than by erosion by water in the past. Two Viking spacecraft were launched towards Mars by NASA in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms. No evidence of life was found, but more recent studies of the Martian landscape suggest that in the past surface water may have been present, which could have enabled life to exist.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This view was taken by the Viking Orbiter 1 spacecraft. These...

This view was taken by the Viking Orbiter 1 spacecraft. These... Photo-6784319.92462 - SFGate

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Mcmurdo Panorama Of Mars, The "Mcmurdo" Panorama Taken By The Mars Exploration Rover Spirit Shows The Area Around "Low Ridge," A Hill In Gusev Crater On Which The Rover Wintered In 2006.

Gully-like features on the slopes of various craters, troughs, and other depressions that may have been formed from liquid water, lay on the surface of Mars in this undated photo. The internet-based space news service www.space.com announced, that if evidence of water is verified, there would be strong implications for the search of life beyond Earth.

Photo: NASA, Getty Images

Gully-like features on the slopes of various craters, troughs, and...

Gully-like features on the slopes of various craters, troughs, and... Photo-6784322.92462 - SFGate

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Planetary bodies. Opaque watercolour on paper, Western India, 18th century. This image shows planetary bodies and the distances between them. Saturn, Mars, Jupiter and other planets are depicted along with suns and moons.

The upper crust of Mars is layered, and inter-bedded with these layers are old, filled and buried meteor impact craters. In a few places on Mars, such as Arabia Terra, erosion has re-exposed some of the filled and buried craters. This NASA Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image released 03 December, 2003 shows an example. The larger circular feature was once a meteor crater. It was filled with sediment, then buried beneath younger rocks. The smaller circular feature is a younger impact crater that formed in the surface above the rocks that buried the large crater. Later, erosion removed all of the material that covered the larger, buried crater, except in the location of the small crater.

Photo: AFP/Getty Images

The upper crust of Mars is layered, and inter-bedded with these...

The upper crust of Mars is layered, and inter-bedded with these... Photo-6784324.92462 - SFGate

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This NASA Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image released 09 December, 2003, shows wind-eroded material, possibly sedimentary rock, among the ridges of the Lycus Sulci region west of Olympus Mons on Mars. The darker surfaces and the dark-toned ripples on those surfaces indicate there may be windblown sand present in these areas.

Scientists At The Johnson Space Center In Houston, Tx., Say February 27, 2001 That A Crystallized Magnetic Mineral, Called Magnetite, Found In A Martian Meteorite Is Similar To Crystals Formed On Earth By Bacteria. "I Am Convinced That This Is Supporting Evidence For The Presence Of Ancient Life On Mars," Said Kathie Thomas-Keprta, An Astrobiologist At The Space Center And The First Author Of A Study In The Proceedings Of The National Academy Of Sciences. Upper Figure: Modern Magnetotactic Bacteria, One Showing A Chain Of Magnetite Crystals, As Seen In The Backscattered Scanning Electron Microscope. Lower Figure: Magnetite Crystals And Chains Of Magnetite Crystals In The Martian Meteorite Alh84001 In The Backscattered Scanning Electron Microscope.. One Conspicuous Chain Indicated By Arrows. The Diameter Of A Single Crystal Ia Approximately One-Millionth Of An Inch. A Crystal Found In A Meteorite From Mars Could Only Have Been Formed By A Microbe And May Be Evidence Of The Oldest Life Form Ever Found, Researchers Say.

Detail of structural form found in 4.5 billion-year-old rock meteorite ALH84001 (believed to be portion of meteorite which fell to Earth from Mars) containing fossil evidence of possible life on planet.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Detail of structural form found in 4.5 billion-year-old rock...

Detail of structural form found in 4.5 billion-year-old rock... Photo-6784333.92462 - SFGate

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Detail of structural form found in 4.5 billion-year-old rock meteorite ALH84001 (believed to be portion of meteorite which fell to Earth from Mars) containing fossil evidence of possible life on planet.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Detail of structural form found in 4.5 billion-year-old rock...

Detail of structural form found in 4.5 billion-year-old rock... Photo-6784335.92462 - SFGate

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Detail of structural form found in 4.5 billion-year-old rock meteorite ALH84001 (believed to be portion of meteorite which fell to Earth from Mars) containing fossil evidence of possible life on planet.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Detail of structural form found in 4.5 billion-year-old rock...

Detail of structural form found in 4.5 billion-year-old rock... Photo-6784336.92462 - SFGate

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Detail of structural form found in 4.5 billion-year-old rock meteorite ALH84001 (believed to be portion of meteorite which fell to Earth from Mars) containing fossil evidence of possible life on planet.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Detail of structural form found in 4.5 billion-year-old rock...

Detail of structural form found in 4.5 billion-year-old rock... Photo-6784337.92462 - SFGate

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Detail of structural form found in 4.5 billion-year-old rock meteorite ALH84001 (believed to be portion of meteorite which fell to Earth from Mars) containing fossil evidence of possible life on planet.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Detail of structural form found in 4.5 billion-year-old rock...

Detail of structural form found in 4.5 billion-year-old rock... Photo-6784338.92462 - SFGate

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Detail of structural form found in 4.5 billion-year-old rock meteorite ALH84001 (believed to be portion of meteorite which fell to Earth from Mars) containing fossil evidence of possible life on planet.

Photo: Time Life Pictures, Time & Life Pictures/Getty Image

Detail of structural form found in 4.5 billion-year-old rock...

Detail of structural form found in 4.5 billion-year-old rock... Photo-6784339.92462 - SFGate

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Orange-colored carbonate mineral globules found in Meteorite ALH84001, believed to be portion of 4.5 billion-year-old Mars rock which fell to Earth 13,000 years ago, containing fossil evidence of possible life on planet.

Mars Global Surveyor Maps Of The Planet. Nasa Topographic Maps Of Mars Released May 27, 1999. The Maps, Which Show High Altitudes As Red, Yellow As Sea Level And Blue As Below Sea Level, Were Created By Pulses Of Laser Light Beamed At Millions Of Points Around The Planet From The Orbiting Mars Global Surveyor Spacecraft And Will Help Nasa Scientists Choose A Landing Site For A Probe Scheduled To Set Down Near The Martian South Pole On December 3.

Nasa Topographic Maps Of Mars Released May 27, 1999. The Maps, Which Show High Altitudes As Red, Yellow As Sea Level And Blue As Below Sea Level, Were Created By Pulses Of Laser Light Beamed At Millions Of Points Around The Planet From The Orbiting Mars Global Surveyor Spacecraft And Will Help Nasa Scientists Choose A Landing Site For A Probe Scheduled To Set Down Near The Martian South Pole On December 3.

During the 1960s and 1970s it was generally considered that the next step in space exploration after the Apollo manned Moon landings would be a manned mission to Mars. This did not materialise and there are no plans to send men to Mars in the foreseeable future. (Photo by SSPL/Getty Images)

Photo: Science & Society Picture Librar, SSPL Via Getty Images

During the 1960s and 1970s it was generally considered that the...

During the 1960s and 1970s it was generally considered that the... Photo-6784344.92462 - SFGate

Dust storms such as this are common on Mars, sometimes covering up to half the planet?s surface. Two Viking spacecraft were launched towards Mars by NASA in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms. No evidence of life was found, but more recent studies of the Martian landscape suggest that in the past abundant surface water may have been present, which could have enabled life to exist. Today Mars has a very thin atmosphere, mainly of carbon dioxide. No liquid surface water appears to exist, but there are permanent polar ice caps, made up of frozen water and carbon dioxide (dry ice).

Photo: Science & Society Picture Librar, SSPL Via Getty Images

Dust storms such as this are common on Mars, sometimes covering up...

Dust storms such as this are common on Mars, sometimes covering up... Photo-6784346.92462 - SFGate

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Close up of the Martian surface, Viking 2 1976. This shows one of the lander's feet and the scrape marks left in the surface by the soil probe. Viking 2 was launched on 9th September 1975 and landed in the Utopia Planitia region of Mars on 3rd September 1976, where it studied the Martian environment, soil constituents and searched for simple life forms - none were found.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

Close up of the Martian surface, Viking 2 1976. This shows one of...

Close up of the Martian surface, Viking 2 1976. This shows one of... Photo-6784347.92462 - SFGate

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This view of the red surface of Mars also shows part of the spacecraft itself. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This view of the red surface of Mars also shows part of the...

This view of the red surface of Mars also shows part of the... Photo-6784349.92462 - SFGate

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This view was taken by one of the Viking Orbiters. The canyon is some 4000 kilometres long and over 6 kilometres deep in places. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This view was taken by one of the Viking Orbiters. The canyon is...

This view was taken by one of the Viking Orbiters. The canyon is... Photo-6784351.92462 - SFGate

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Part of the Viking lander spacecraft can be seen in this picture. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

Part of the Viking lander spacecraft can be seen in this picture....

Part of the Viking lander spacecraft can be seen in this picture.... Photo-6784352.92462 - SFGate

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This shows a flat rock-strewn surface with a pink sky. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 1 was launched on 20th August 1975 and landed in the Chryse Planitia region of Mars on 20th July 1976. It continued to return data to Earth until November 1982.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This shows a flat rock-strewn surface with a pink sky. Two Viking...

This shows a flat rock-strewn surface with a pink sky. Two Viking... Photo-6784353.92462 - SFGate

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Part of the spacecraft can be seen in this picture. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 1 was launched on 20th August 1975 and landed in the Chryse Planitia region of Mars on 20th July 1976. It continued to return data to Earth until November 1982.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

Part of the spacecraft can be seen in this picture. Two Viking...

Part of the spacecraft can be seen in this picture. Two Viking... Photo-6784354.92462 - SFGate

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The image shows a red rock-strewn terrain. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 2 was launched on 9th September 1975 and landed in the Utopia Planitia region of Mars on 3rd September 1976.

The image shows a red rock-strewn terrain and part of the lander itself. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 2 was launched on 9th September 1975 and landed in the Utopia Planitia region of Mars on 3rd September 1976.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

The image shows a red rock-strewn terrain and part of the lander...

The image shows a red rock-strewn terrain and part of the lander... Photo-6784356.92462 - SFGate

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The image shows a red rock-strewn terrain and part of the lander itself. The trench dug by the soil retrieval scoop can be seen in the centre of the picture. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 2 was launched on 9th September 1975 and landed in the Utopia Planitia region of Mars on 3rd September 1976.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

The image shows a red rock-strewn terrain and part of the lander...

The image shows a red rock-strewn terrain and part of the lander... Photo-6784358.92462 - SFGate

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This shows a sample of soil actually in the lander's soil retrieval scoop. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 1 was launched on 20th August 1975 and landed in the Chryse Planitia region of Mars on 20th July 1976. It continued to return data to Earth until November 1982.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This shows a sample of soil actually in the lander's soil retrieval...

This shows a sample of soil actually in the lander's soil... Photo-6784359.92462 - SFGate

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The image shows a red rock-strewn terrain with a pink sky. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found.

This shows many images of the colour scale on the lander, used to calibrate the pictures, against the Martian surface. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This shows many images of the colour scale on the lander, used to...

This shows many images of the colour scale on the lander, used to... Photo-6784361.92462 - SFGate

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This shows the red rock strewn surface of Mars and the shadow of the lander itself. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both successfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found. Viking 2 was launched on 9th September 1975 and landed in the Utopia Planitia region of Mars on 3rd September 1976.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

This shows the red rock strewn surface of Mars and the shadow of...

This shows the red rock strewn surface of Mars and the shadow of... Photo-6784362.92462 - SFGate

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These images of Mars were taken using a webcam attached to a 180-cm Maksutov-Newtonian telescope. Photograph by Jamie Cooper.

Photo: Jamie Cooper, SSPL Via Getty Images

These images of Mars were taken using a webcam attached to a 180-cm...

These images of Mars were taken using a webcam attached to a 180-cm... Photo-6784363.92462 - SFGate

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After driving a short distance from their landing site, two explorers stop to inspect a robotic lander and its small rover in this artist's concept of a future Mars mission. This stop also allows the crew to check the life support systems of their rover and space suits within walking distance of the base.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

After driving a short distance from their landing site, two...

After driving a short distance from their landing site, two... Photo-6784364.92462 - SFGate

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Two Viking spacecraft landed on the surface of Mars in 1976. Viking 1, launched on 20th August 1975, touched down in the Chryse Planitia region on 20th July 1976, and Viking 2, launched on 9th September 1975, landed in the Utopia Planetia region on 3rd September 1976. They returned many such pictures of the Martian surface as well as carrying out experiments to search for life. No life forms were found.

Photo: Science & Society Picture Librar, SSPL Via Getty Images

Two Viking spacecraft landed on the surface of Mars in 1976. Viking...

Two Viking spacecraft landed on the surface of Mars in 1976. Viking... Photo-6784365.92462 - SFGate